Hi,
Your message is quite old, so I think it’s too late…
But I think some people get the same issue (like me), so I wrote a simple script to deal with it :
http://www.devblog.fr/2011/05/16/plusieurs-fonds-personnalises-wordpress-add_custom_background/
It allows to declare as many backgrounds as you need, and then explain how to dispatch them.
Hope this will be helpful !
